The vast majority of, if not all, air conditioning systems in use today operate in accordance with the Rankins cycle in which a working gas, normally a fluorocarbon, is compressed to a liquid state by a compressor and circulated through condenser coils under high pressure to dissipate the heat of compression to the external ambient. The cooled compressed working fluid is then circulated under low pressure through evaporator tubes where it evaporates and extracts heat from the air to be cooled. After being evaporated, the working gas is recirculated back to the compressor where it is compressed again to begin the next cycle.
Although such air conditioning systems have been in extremely wide use for many years they continue to have severe problems. Air conditioning systems are one of the least reliable and most inefficient systems in a vehicle. The leakage of the fluorocarbon working gas is a particularly growing concern with all air conditioning systems since it has been recognized in recent years as a major contributor to the destruction of the stratospheric ozone layer. As a result, efforts are growing to minimize and eventually eliminate the use of fluorocarbons to the extent possible.
Such air conditioning systems have additional severe, inherent disadvantages. The air to be cooled is blown past the evaporator coil by a blower fan to cause the expansion of the working fluid in the coil to extract heat from the air. The expansion of the working fluid in the evaporator requires a large volume so that the evaporator tube must be quite long and occupies a large cross-sectional area. The heat from the air to be cooled must be transferred from the air to the aluminum fins that are typically affixed to the evaporator tube, from the fins to the copper evaporator tube, and from the tube to the working fluid. This 4-step process is inherently inefficient. If the fins are not present as they are not in many instances, the heat exchange is even less efficient. Because the heat exchanger is inefficient, the temperature of the evaporator tube must be far below that to which the air is to be cooled. This results in excessive condensation of moisture from the air onto the tube which further reduces efficiency. In addition, the collection of water on the tube, especially at locations shielded by the tube from the flow of air, can cause icing that can ultimately disable the system. While certain amount of dehumiiification of the air is beneficial, excessive dehumidification can be unhealthful.
In large vehicles, such as buses, the evaporator coils are positioned at one location in the bus, normally at the back or on the roof. Thus, the cooled air must be conducted through ducts for substantial distances to cool remote portions of the vehicle which results in further losses of cooling efficiency.
Such large vehicle air conditioning systems are also inflexible in that there is one evaporator coil and all the air being circulated is cooled to the same degree. Thus the system is unable to compensate for variations in the required cooling load in different parts of the bus as the bus moves. For instance, if direct sunlight hits one side of the bus while the other side is shaded, the cooling load to maintain the sunny side of the bus at a particular temperature is substantially higher than that on the other side. As the bus travels, the cooling loads in different parts of the vehicle change continually depending on its orientation with respect to the sun and other factors. A special cooling loop is sometimes provided for the driver alone.
Another energy wasting consequence of the limitations of available vehicle air conditioning systems arises from the fact that when the driver has to wait for extended periods on a warm day, he will usually keep the motor and the air conditioning system running to cool the entire bus.
In addition to these difficulties, vehicle air conditioning systems are generally heavy, bulky and noisy and require major maintenance at frequent intervals.. Thus they adversely affect fuel economy, vehicle design flexibility and reliability and passenger comfort.